Rod-shaped force transducers are known, in which a longitudinal strain and a transverse strain of a rod-shaped measuring body or deformation body are detected, respectively with the aid of strain transducers applied on the deformation body. From the electrical signals produced by the strain transducers, based thereon a force to be measured can be determined. When they are provided for the construction of weighing scales, such force transducers are also referred to as load cells or weighing cells.
For a rod-shaped force transducer of the described type, a costly or time-consuming compensation adjustment is necessary, which is also referred to as a rotation compensation. Thereby, tolerances of the deformation body or of the strain transducers that arise due to the production are to be counteracted, and the force transducer is to be made less sensitive to lateral or transverse force influences and therewith also to changes in the force introduction.
The rotation compensation is carried out as follows. One end of the rod-shaped force transducer is clamped into a support or holding device. At its other end, the force transducer is loaded by a loading device. This process is repeated for another three times after a respective rotation of the holding device together with the clamped-in force transducer by 90°. From the measured values of the force transducer in the various different rotational positions, a conclusion is reached whether a compensation is necessary. If that is the case, then the force transducer is again rotated several times and a mechanical compensation by material removal and/or an electrical compensation is carried out. Alternatively, the compensation can also be carried out in that not the force transducer but rather the loading device is rotated. In this case, several material removal devices or a rotatable material removal device are necessary for the mechanical compensation.
The rotational compensation always requires several rotations and loadings. Therefore carrying out the rotational compensation is costly or time-consuming.
In the EP 0 800 064 B1, a rod-shaped force transducer of the above described type is explained, and the rotational compensation carried out for it is mentioned. In the DE 44 16 442 A1, the rotational compensation is described in further detail.
Even after such a rotational compensation, the danger of a falsification of the measurement results still exists due to the following effect. If a force is introduced not fully coaxially to the longitudinal axis of the rod-shaped embodied deformation body, via the force introduction surfaces on the end face ends of the deformation body, and/or additional transverse forces are effective, then the deformation body becomes deformed in the direction transverse to the longitudinal axis, not uniformly, but rather in a one-sided manner. In other words, the deformation body tilts toward one side and becomes deformed non-coaxially to its longitudinal axis, whereby the lines of force no longer extend in the desired manner. This leads to a falsification of the measurement results.
Besides the rotational compensation, e.g. also a temperature compensation is carried out, so that in total several compensation measures must be carried out for the known rod-shaped force transducers of the above described type.
A rod-shaped force transducer with a cylindrical compression body is also known from the GB 2 162 322 A, in which a force to be detected bears upon the end faces of the compression body. It comprises two elongated holes that are oriented longitudinally along the force direction and lie opposite one another, of which the base surfaces form a web onto which strain gages arranged longitudinally and transversely to the force direction are applied. The web comprises a through-going bored hole respectively in front of and behind the strain gages in the force direction. Necessitated by the two bored holes, under the influence of a force, the lines of force extend past the bored holes laterally next to the two bored holes. Thereby, the force line distribution in the middle of the compression body, where the strain gages are located, also becomes increasingly inhomogeneous. The larger the diameter of the two bored holes, and the smaller the spacing distance of the bored holes from the strain gages, the fewer the force lines that will extend through the area of the web lying between the strain gages. The sensitivity of the force transducer can be increased in that the two elongated holes are made deeper and/or wider. It can be reduced in that the diameter of the two bored holes is increased.
In the force transducer described in this GB 2 162 322, the measuring behavior is influenced by material removal in order to enlarge the measuring range with a linear characteristic curve. Thereby the sensitivity of the force transducer is increased, in that the two elongated holes are made deeper and/or wider, and is reduced in that the diameter of the two bored holes is increased. In this manner the measuring range with a linear characteristic curve is to be adjusted very exactly. This is an iterative and costly or time-consuming process.